Extrusion coating is a known method of directly depositing process coatings onto substrates, wafers, flat panel displays, and similar objects (collectively “substrates”) in the microelectronics and display technology industries. According to a typical prior art system, substrates are transported linearly beneath an extrusion coating head, and process fluids are precisely dispensed from a linear orifice in the extrusion head using a microprocessor-based electrohydraulic pumping system. One such system is described in U.S. Pat. No. 4,696,885 entitled “METHOD OF FORMING A LARGE SURFACE AREA INTEGRATED CIRCUIT”. Depending on the particular application, such process fluids include photoresist, polyimides, color filter materials and the like. Such extrusion coating techniques are well-suited for research and development activities as well as high volume production requirements.
Although known extrusion systems of this type provide significant advantages as compared to other liquid deposition techniques (such as spin coating), they often suffer from a similar problem, specifically, the inability of the coating head to establish a uniform coating at the leading edge of the substrate during certain applications. In these systems, each substrate is a discrete part unlike a web coating process, and the coating deposition is therefore started and stopped with each new substrate. With such part by part processing, a coating “bead” must be re-formed between the extrusion head and each new substrate to thereby “wet” the surfaces. When this bead initially contacts the substrate, however, it may cause a “perturbation” for some measurable distance (e.g., 5–20 mm) from the leading edge of the coating. Sometimes a leading edge anomaly of this type dictates that the substrate be rejected completely, thus increasing material and process costs and decreasing process efficiency.
There have been attempts in the art to address the problem of establishing a uniform coating condition in a linear or so-called slot type extrusion coater, and systems of this type are illustrated in U.S. Pat. Nos. 4,938,994 entitled “METHOD AND APPARATUS FOR PATCH COATING PRINTED CIRCUIT BOARDS” and U.S. Pat. No. 5,183,508 titled “APPARATUS FOR PATCH COATING PRINTED CIRCUIT BOARDS”. In these patents, a controlled volumetric flow rate of the liquid is delivered to a liquid containing chamber within the extrusion head and then through the applicator slot to create what is said to be a uniform volumetric flow rate of liquid exiting from each point along the slot. A displacement piston associated with the extrusion head generates a fluid pulse to control the formation of a connecting bead of the liquid coating prior to, at the same time as, or after the sending of the controlled volumetric flow rate of the liquid. This technique purports to apply a layer of the liquid with a precisely-controlled volume per unit area of the liquid to the substrate. Prior art machines also include a slot sealing unit that cleans the extrusion head slot between applications. The prior art systems however, do not effect a condition for the extrusion head at the beginning of a coating operation which mimics the head's condition in the middle of a coating operation. Therefore the problem of a gradual drift towards a steady state coating rate remains, resulting in leading edge perturbations. Therefore, prior art techniques do not adequately address the problem of leading edge perturbations that may affect the uniformity of the coating.
In order to avoid dripping or smearing coating material which has gathered around the extrusion head after a coating operation, it is often necessary to clean the extrusion head before a new coating operation begins. In the prior art, cleaning of extrusion mechanisms is usually accomplished manually, potentially leading to inconsistent results and disruption and delay of the coating operations. The presence of residual coating material on the extrusion during a coating operation can result in unwanted deposition of coating material, or contaminants which have collected on the residual material, on the substrate and/or on part of the coating apparatus. Therefore, it is a problem in the art that manual cleaning operations are inconsistent and unreliable.
Therefore there is a need in the art for a mechanism to overcome the problem of leading edge anomalies arising during the slot type coating of substrates in a batch process.
There is a further need in the art for a mechanism which will effectively and consistently clean an extrusion head so as to prevent dripping coating material onto a substrate or other surface.